1. Field of the Invention
The present invention relates generally to a method of data transmission for transmitting via a clocked transmission device binary-coded data incorporated in block form into a time frame structure with constant frame length.
2. Description of the Related Art
Transmission systems which operate by the PCM-2 (PCM=Pulse Coded Modulation) method use a transmission technique that is customary in the case of ISDN networks (ISDN=Integrated Services Digital Network) to transmit two voice channels from the exchange, such as a telephone exchange, to the subscriber on a pair of copper wires. The two 64-kbit/s B channels which are available with ISDN systems are connected via a corresponding adapter in the exchange to two ports of an analog line termination circuit and at the subscriber to two analog telephones. In this case, which corresponds to analog telephones, each of the two subscribers is provided with a 64-kbit/s channel. Consequently, each subscriber can use fast fax machines without any loss in power.
If, in addition, the voice data are compressed during transmission, the capacity of an ISDN transmission device is available to a still greater number of analog subscribers. However, transmission by fast fax machines unfortunately suffers.
Transmission in accordance with the HDSL standard (ETSI RTR/TM 3036) provides a frame length of 6 ms, wherein the information is reduced from two bit lengths to a quaternary symbol, in other words, a so-called quat, during transmission. This results in a bit rate which is equal to twice the band rate. The nominal length of an HDSL frame is 6 ms. The number of quats per HDSL frame depends on the respective operating mode, two operating modes (namely the E1 and T1 operating modes) being standardized. The data within an HDSL frame are divided over four blocks, so-called payload blocks, with in each case 12 groups, which begin with a start bit (a Z bit), which is followed by a number of payload bits. Since 4.times.12 z bits are transmitted in a 6-ms period, there is additionally an 8-kb/s channel available. The number of bits transmitted after every Z bit is equal to the number of 64-kb/s channels which are transmitted over a line. Consequently, the operating mode T1 comprises 12 bits after every z bit, while the operating mode E1, which does not have the 8-kb/s channel, has 18 bits after every Z bit.
The average length of an HDSL frame in a T1 operating case is 2352 quats in 6 ms. Each individual frame contains either no filling quats or two filling quats, 6-1/392 ms or 2353 quats in 6+1/392 ms. The average length of the HDSL frame for the E1 operating case is 3504 quats in 6 ms. Each individual frame in this case comprises no filling quats or two filling quats, so that an actual length of 3503 quats in 6-1/584 ms or 3505 quats in 6+1/584 ms is obtained. PCM-11 systems use, for transmission in accordance with the HDSL standard, 11 our of 12 B channels for 11 analog subscribers and the 12th B channel is used for signaling information. In this case, the 11 subscribers enjoy the full 64-kbit/s service. However, the remote feeding of the power for the terminal devices is problematical here. Although the data rate is increased by use of the HDSL standard, the power which can be transmitted for feeding to the terminal devices is limited owing to the series resistance of the line. For example, in the case of a PCM-11 system, 11 time the power required by a subscriber station plus the power which the PCM system itself requires, would have to be provided. Consequently, in some applications there are limitations to the line length which are caused only the feed voltage that is available in the exchange. Here, other data rates, for example 5 or 9 B channels--would be desirable in order, on the one hand, to minimize the power consumption and, on the other hand, to optimize the line length or connect the maximum number of subscribers to a given connection.
So far it has only been possible by means of transmission in accordance with the HDSL standard to realize PCM-n systems (for values of 4.ltoreq.n.ltoreq.11) with the full 64-kbit/s data rate per subscriber. The remaining channels between 12 or 18 and n were unused in these cases. The power consumption of the system was consequently not at an optimum and the achievable range was only that of the PCM system with 12 or 18 channels. In this case, the range decreases as the data rate increases.